Optical signal dispersion compensators (DCs) can correct for chromatic dispersion in optical fiber and are especially useful for bit rates 10 Gb/s and higher. Furthermore, it is advantageous for the dispersion compensator to have an adjustable, also called “tunable”, amount of dispersion, facilitating system installation. It is also advantageous if the tunable dispersion compensator (TDC) is colorless, i.e., one device can compensate many channels simultaneously or be selectable to compensate any channel in the system.
Previously proposed colorless TDCs include ring resonators, the virtually imaged phased array (VIPA), cascaded Mach-Zehnder interferometers (MZIs), temperature-tuned etalons, waveguide grating routers (WGRs) with thermal lenses, and bulk gratings with deformable mirrors.
In my recently filed application entitled “TUNABLE DISPERSION COMPENSATOR” filed on Jan. 20, 2004, Ser. No. 10/760,516, now U.S. Pat. No. 6,961,492, I described a method and apparatus for implementing a colorless Mach-Zehnder-interferometer-based tunable dispersion compensator. While this TDC achieves a large dispersion range with a very simple design, it has a drawback in that it has a very narrow optical bandwidth. It can tolerate a misalignment between the wavelength of the transmitter and a center wavelength of the TDC, within one of the TDC free-spectral ranges, of about +/−20 pm. This TDC optical bandwidth is acceptable for wavelength-locked transmitters, but many applications use non-wavelength-locked transmitters, also called “TDM” transmitters. TDM transmitters usually have a wavelength drift specification of +/−100 pm over their lifetime, which may be too large for the TDC optical bandwidth.
To overcome this optical bandwidth limitation and to accommodate for a TDM transmitter wavelength drift specification, I disclosed locking the TDC to the TDM transmitter laser wavelength by adjusting phase shifters in the two outermost MZIs of the three MZI stage TDC. For instance, by increasing the drive to phase shifters in both longer arms of the two outermost MZIs in unison, the TDC can be tuned to longer wavelengths. The feedback control mechanism for the locking is derived by dithering these phase shifters in the outermost MZIs in unison at a specific frequency and measuring the output power from the TDC using a tap and a photodetector, employing a standard peak-detection feedback control.
However, the use of dithering undesirably adds optical modulation to the data modulated wavelength signal being transmitted through the TDC and over the system, which may affect the ability of a receiver to detect the data modulated on that wavelength signal. Additionally, since data modulation of the wavelength signal appears as dithering, it would adversely cause the TDC to change its center wavelength and affect dispersion compensation. Additionally, such a dithering technique does not adequately work for low dispersion settings.
Thus, there is a continuing need for a TDC that has both a large dispersion range and which can accommodate for a TDM transmitter wavelength drift specification that can vary +/−100 pm over its lifetime.